High pressure decorative laminates are conventionally produced by stacking and curing under heat and pressure a plurality of layers of paper impregnated with various synthetic thermosetting resins. In normal practice the assembly, from the bottom up, consists of a plurality, e.g. three to eight, core sheets made from phenolic resin impregnated kraft paper, above which lies a decor sheet impregnated with melamine resin; on top of the decor sheet is provided an overlay sheet which, in the laminate, is almost transparent and provides protection for the pattern sheet.
The core sheets are conventionally made from kraft paper of about 90-155 pound ream weight. Prior to stacking, the kraft paper is impregnated with a water-alcohol solution of phenolformaldehyde resole, dried and partially cured in a hot air oven, and finally cut into sheets.
The decor sheet is a high quality, 50-125 ream weight, pigment filled, alpha cellulose paper that has been impregnated with a water-alcohol solution of melamine-formaldehyde resin, dried and partially cured, and finally cut into sheets. The decor sheet, prior to impregnation with the resin, usually has been printed with a decorative design, or with a photo-gravure reproduction of natural materials, such as wood, marble, leather, etc., but it may also be solid colored.
The overlay sheet is almost invariably used when the decor sheet has a surface printing in order to protect the printing from abrasive wear. The overlay sheet is a high quality alpha cellulose paper of about 15-35 pounds ream weight that is also impregnated with melamine-formaldehyde resin in a manner similar to that used for the decor sheet, except that a greater amount of resin per unit weight of paper is used. The individual sheets are stacked in the manner indicated above and, if six sheets of impregnated core paper are used, there results a finished laminate having a thickness of about 50 mils, it being understood that a different number of sheets can be used to provide thicker or thinner laminates.
The stack of sheets as described above is placed between polished steel plates and subjected to about 230.degree.-340.degree. F. (e.g. 300.degree. F.) at 800-1600 p.s.i. (e.g. 1000 p.s.i.) for a time sufficient to consolidate the laminate and cure the resins (e.g., about twenty-five minutes). This causes the resin in the paper sheets to flow, cure and consolidate the sheets into a unitary laminated mass referred to in the art as a decorative high-pressure laminate. In actual practice, two laminated stacks are pressed back to back, separated by a coated release sheet that allows the two laminates to be peeled apart after separation. Also, a large proportion of the stacks are laminated with an aluminum foil-kraft paper composite sheet inserted between the overlay and the metal plate, with the aluminum facing the overlay, in order to obtain a laminate having a lower gloss and a slightly textured surface which is desirable for some products.
At the completion of the laminating operation, the backs of the laminates are sanded to permit gluing to particle board, plywood or other substrates. The glued, laminate surfaced panel is then fabricated into furniture, kitchen counter tops, table tops, store fixtures and other end-use applications widely accepted for the combination of appearance, durability and economy.
A number of variations of the above-described general process are known, particularly those operations designed to obtain special effects in appearance and texture. Also other curing cycles are possible and, in fact, sometimes other resin systems are used as well.
Besides decorative high-pressure laminates referred to above, there are also a number of low-pressure products which have been developed in recent years, including low-pressure laminates using either saturated polyester resins, or melamine-formaldehyde resin. One of the fastest growing materials competing with high-pressure laminates in recent years is a product referred to as low-pressure melamine board which is normally pressed in a short cycle at 175-225 p.s.i. at 325.degree.-350.degree. F. These low-pressure products have the advantage of being normally less expensive, but they cannot be given the title of "high pressure laminates" because such a product must meet a variety of rigid standards promulgated by the National Electric Manufacturers Associates, NEMA LD3-1980, which include standards relating to abrasive wear, stain resistance, heat resistance, impact resistance, dimensional stability, etc. While various other decorative printed, surfacing materials, such as some of the low-pressure laminates, have certain of the desirable characteristics, no products other than high-pressure laminates currently available have all of these properties.
One of these properties in particular which is very important is abrasion resistance. A high-pressure decorative laminate must have sufficient abrasion resistance to permit use in high exposure areas such as dinette surface tops, check-out counters, etc. The standard NEMA test for abrasion resistance is NEMA test LD-3.01. In this test, a laminate sample is clamped on a rotating disc, over which ride two weighted rubber wheels, faced with calibrated sand-paper strips. As the laminate surface is rotated under the wheels, the abrasive action of the sand paper cuts through the surface of the laminate and gradually through the overlay until the printed pattern is exposed and destroyed. The NEMA standard for Class I laminate requires that the laminate, after four hundred rotation cycles, has no more than 50% of its pattern destroyed. The 50% end point is estimated by averaging the number of cycles at which the pattern shows initial wear, and the number of cycles at which the pattern is completely destroyed.
If a high-pressure decorative laminate is prepared in a conventional manner, with a normal 35-40% resin content in the decor sheets, but without an overlay sheet, the abrasion resistance will be only about 50-75 cycles. If specially formulated melamine resins are used in the decor sheet with a resin content of 50-55%, abrasion resistance of up to about 150-200 cycles are on occasion obtainable without an overlay sheet, but in this latter case the laminates have a tendency to develop surface craze and, furthermore, they are quite difficult to prepare due to the difficulty of impregnating the decor sheet in a uniform manner; additionally, they do not meet the 400 cycle minimum required by the NEMA standard.
Very significant improvements to the conventional system described above are disclosed in Scher et al., U.S. Pat. Nos. 4,255,480 and 4,263,081. These patents and their progeny disclose the production of highly abrasion resistant laminates without overlay, the decor sheet being coated with an ultra-thin layer of small abrasion-resistant particles, e.g. alumina, immobilized in place on the decor sheet by a suitable binder material, most desirably micro-crystalline cellulose.
The elimination of the overlay sheet produces very significant economic advantages. In addition, the resultant laminates have superior abrasion resistance, meeting not only the highest NEMA abrasion resistance standards, but also having both superior "initial wear" resistance and superior abrasion resistance to rubbing and sliding cans.
Besides resistance to abrasion and wear, it is necessary that decorative laminates, especially "high-pressure laminates," have surfaces characterized by resistance to heat, stain by common household chemicals and foods, impact and boiling water, all consistent with the NEMA LD3-1980 standards.
In addition to the above, another source of damage to a laminate surface is scuffing. Prior to the actual installation of a laminate surface, the laminate sheet must be handled many times, cut to size, bonded to a substrate, cut to final size, edged, and then finally moved to a final location and installed. During all this handling, the laminate surface can come into sliding contact with surfaces of other sheets, corners of sheets, and corners of edges of other materials. During these various sliding contacts, the laminate surface can gain unsightly scuff marks, which are not distinctly scratches or cuts in the laminate surface. The scuffs often appear to be a deposit of material from the object rubbing in contact with the laminate surface. For example, if a white colored laminate is rubbed by the corner or edge of a blue colored laminate, the scuff mark appears to have embedded therein blue particles. In this sense, the scuff mark is analogous to the process known as "galling" when two similar clean surfaces of metals are rubbed together at high pressure. It is well known to metallurgists that galling represents a transfer of metal from one surface to the other.
Some scuffs do not have an obvious transfer of material from an offending contacting object. In these cases, the area of the scuff appears to have been burnished to a higher gloss, or dulled to a lower gloss. This type of scuff appears to be caused by a very small amount of flow in the laminate surface due to the pressure of contact with the corner or edge of the offending object. Most frequently, however, the scuffs appear to be caused by a combination of the two effects just described, i.e. galling together with burnishing or dulling.
In the past, most decorative laminates were produced in light colors for work tops and other large area uses. Dark or intense colors could not be used in large areas because of the scuffing problem and therefore were most often used in smaller areas for color accents. Within the past ten years, however, the demand for dark and intense colors has greatly increased, even for large area uses such as the surfaces on store fixtures, elevator cabs, restaurant tables. As scuff marks of the kind described are much more visible when they occur on dark or intensely colored surfaces, than on light colored surfaces, they are much more objectionable to the owners of the installation. This has caused a significant amount of customer dissatisfaction, and considerable expense to the manufacturers and fabricators of laminate for replacement of defective installations.
While the laminates of the aforementioned Scher et al., U.S. Pat. Nos. 4,255,480 and 4,263,081, are much improved in abrasion resistance, initial wear resistance and rub resistance, they are quite subject to scuffing, particularly the "galling" type described above, and this is so because their surfaces can act, at the microscopic level, like sand paper, i.e. the hard particles at the surface of the decor sheet can actually protrude above the surface of the finished laminate. This problem is enhanced when the decor sheet is solid, dark colored.
Accordingly, it would be advantageous to provide a decorative laminate which, while maintaining high abrasion resistance, avoids and reduces the problem of scuffing.